Method of forming a tubular blank into a structural component and die therefor

ABSTRACT

A method of forming an elongated tubular blank into a tubular structural component having a predetermined outer configuration, the method comprising: providing a shape imparting shell formed from a low permeability, rigid material which includes an inner surface defining the predetermined shape, plugging the open ends of the tubular blank, placing the plugged blank into the shell, and forming the tubular blank into the tubular component by inductively heating axial portions of the blank by axially spaced conductors adjacent the shell while or before forcing gas at a high pressure into the plugged blank until the blank conforms to at least a portion of the inner surface of the shell to form the structural component.

This application claims the benefit of copending provisional applicationNo. 60/155,969 filed Sep. 24, 1999.

The present invention relates to the art of forming structuralcomponents such as used in motor vehicles and more particularly to amethod of forming a tubular blank into a structural component by use ofhigh pressure inert gas.

INCORPORATION BY REFERENCE

The invention involves formation of tubular metal components ofstructural type as used in automotive support frame where a tubularblank is formed to match the shape defined by the inner surface of ashell or cavity. In accordance with the invention, the shell or cavityis in a low permeability cast support structure wherein inductionheating coils are embedded for inductively heating the tubular blankpreparatory to formation into the desired shape imparted by the shell orcavity. A similar technology has been developed by Boeing Companywherein a flat plate is formed against a contoured wall by gas pressure.This process is referred to as superplastic forming of the plate and isdisclosed in Gregg U.S. Pat. No. 5,410,132, incorporated by referenceherein. This patent illustrates a process whereby the temperature of themetal plate is increased to a superplastic temperature by inductionheating conductors mounted in a ceramic, low permeability cast diesurrounding the metal forming chamber defined between two dies. This gaspressure chamber includes one surface against which the plate is formed.The Boeing process, as disclosed in Gregg U.S. Pat. No. 5,410,132,utilizes induction heating coils for the purposes of heating the metalpreparatory to forming against a shaped surface by using high pressuregas on one side of the plate. The extent to which the Boeing patentdefines a ceramic die with embedded induction heating coils and the useof a high pressure inert gas for forming the metal sheet, the technologyrelates to the technology employed in the present invention. For thatreason, details of the die induction heating coils and high pressure gasforming may not be repeated to understand the present invention. InMatsen U.S. Pat. No. 5,530,227, Boeing Company further illustrates moredetails about the die, induction heating coils in a cast die formingmaterial and the dies used by Boeing Company for superplastic forming ofa sheet metal plate. Matsen U.S. Pat. No. 5,530,227 is also incorporatedby reference herein so that the details of the technology developed byBoeing Company and usable in the present invention need not be repeated.Hot metal gas forming of steel is generally described in a joint ventureproposal to the National Institute of Standards and Technology on Mar.18, 1998. The proposal is incorporated by reference herein as backgroundinformation.

BACKGROUND OF INVENTION

The present invention is primarily directed toward the production ofstructural components of the type used in the automotive field and itwill be described with particular reference thereto; however, theinvention is much broader and may be used for forming various structuralcomponents from tubular sheet metal blanks. In the past, such structuralcomponents were normally produced by stamping, forming and welding. Inan effort to obtain complex shapes, such components have been formed bya hydroforming process wherein tubular blanks are provided from sheetsteel material having specific initial strength and elongation. Thetubular blank is cut to length and pre-bent or preformed into a shapeapproximating the shape of the finished structural component. Thepreformed tubular element is loaded into a two piece die closed in ahydraulic press typically having a closing pressure between about3500-8000 tons. The exposed ends of the tubular blank are sealed and thetube is filled with a water and oil mixture. The internal pressure ofthe water and oil mixture is raised to a high level in the generalneighborhood of 20,000-80,000 psi which pressurized liquid expands thetubular blank into the shape of a steel die cavity machine in two diemembers of a die set carried by the hydraulic press. The cavities of thetwo die members have the desired final shape for the structuralcomponent so that as the tubular blank is expanded into the cavity, theouter shape of the component captures the shape of the cavity. Thisprocess produces a relatively accurate complex outer shape for thestructural component. To relieve the fluid pressure, holes are piercedinto the formed structural component. Thereafter, the two die membersare opened by the hydraulic press and the liquid is drained from theformed structural component. Secondary machinery operations, such astrimming and cutting mounting holes, is then performed to produce adesired component for final assembly. This process is gaining popularitybecause it forms the component from the inside so complex shapes arepossible; however, the total cycle time for hydroforming is at leastabout 25-45 seconds. The equipment to direct high pressure liquid intothe tubular blank is extremely large and expensive. In addition, the diemembers are expensive machined parts and have a short life. Hydroformingoperations have a general limitation that they are used primarily forbending of the tubular blank, since the steel being formed is processedat ambient temperature which limits the maximum strain rate for themetal being formed. Pressure of the liquid used in the hydroforming mustbe extremely high to deform the relatively cold sheet metal of thetubular blank into simple configurations. Consequently, hydroforming isused primarily for bending and straightening tubular elements into thedesired final shape. Even though there are process limitations in usinghydroforming to make tubular structural components, a substantialtechnology field has developed around this process. In a feature ofhydroforming, the sheet steel tubular blank is formed into desiredshapes while additional material is forced axially into the die cavityso the wall thickness does not drastically decrease as the volume of agiven cross section increases during the processing by high pressureliquid.

Hydroforming is the primary prior art constituting the background of thepresent invention. However, blow forming of plastic sheets has been usedfor years to produce high volume plastic containers using conventionalsteel die members. Of course, such die members used in plastic blowforming can not be used for forming steel. For that reason, hydroformingis used for metal, instead of blow forming as used in the plasticsindustry. The highly developed technology of hydroforming of steel tubesand blow forming of plastic sheets are background of the presentinvention, but are not economically usable for forming sheet steeltubular blanks into tubular structural components. In addition, theseprior processes do not have the capability of controlling themetallurgical characteristics along the length of the tubular blank, asobtainable by the present invention.

Even though hydroforming of sheet steel and blow forming of plasticsheets are the basic background to the present invention, it has beenfound that certain features of the technology disclosed by BoeingCompany for superplastic forming sheet metal plates by high pressure gasare used in practicing the invention. The Boeing Company process is notbackground information from the standpoint that it is not capable offorming a tubular metal blank into a structural tubular component and isnot capable of controlling the metallurgical characteristics of themetal forming the structural tubular component. These are all advantagesof the present invention.

SUMMARY OF INVENTION

The present invention provides a completely different type of technologywhich is dissimilar to hydroforming of steel and blow forming of plasticsheets. In accordance with the present invention, a tube is made fromsheet metal formed by controlled rolling of the sheet. The sheet metalis formed into a tubular blank which is preheated using resistanceelectric heating and preformed to the desired axial profile. Thepreheated blank is placed into the shell or cavity of a speciallyconstructed die set in which are embedded induction heating conductorsor coils spaced axially along the cavity. The tubular blank has an openend or open ends which are plugged or sealed. The tubular blank isexpanded by high inert gas at a pressure in the general range of100-5,000 psi, but preferably in the range of 200-1000 psi. Duringexpansion, the induction heating conductors or coils induce an A.C.voltage into the metal of the blank which cause I²R heating of theblank. Consequently, the blank can be rapidly expanded. The cavity orshell having the desired predetermined shape surrounds the expandingtube to impart, to the outer surface of the blank the shape of the shellor cavity. The structural element is then cooled at a controlledquenching rate to control the metallurgical characteristics to enhancethe mechanical properties of the resulting structural components.

By using the present invention there is developed a new metal formingprocess technology that reduces the cost to process tubular structuralcomponents by at least 50% and reduces the time to build, and the costto build, the forming die members by at least about 40%. By usingstructural components formed by the unique process of the presentinvention, the structural component is reduced in weight byapproximately 20%. Although the inventive method involves the use of gasto expand the sheet metal tubular blank into the desired configurationfor the structural element, the invention actually involves substantialimprovements in this general process. In other words, the presentinvention is not merely the use of high pressure inert gas as asubstitute for high pressure liquid used in hydroforming. One aspect ofthe invention involves the formation of a unique cavity or shell whichis mounted in the die members of the die set opened and closed by thehydraulic press. The shells and die members are constructed so that thetubular blank being formed into the shape of the shell or cavity can beheated inductively along its length to control the heat of the tubularblank before and during the forming process. This can not be done inhydroforming. By using induction heating in the tools or die members,the heating conductors or coils can localize the heating along thelength of the blank. The die set not only supports induction heatingconductors, but also (a) supports the forces necessary to restrain thetubular blank being formed and (b) provides increased wear resistance.By using the present invention, the yield strength along the length ofthe resulting component or end product is varied by proper heating andcooling. This is particularly advantageous if extended deformation isrequired in producing the desired finished shape of the structuralelement.

By using the present invention, a tubular structural component can beformed having more detailed outer configurations than obtainable withhydroforming. Indeed, the invention obtains the result generallyassociated with blow forming plastic sheets, but for sheet metalcomponents. This is accomplished by utilizing a unique and novelmaterial from which the die member containing the forming cavity isconstructed so induction heating along the length of the tubular blankcan be varied. Consequently, the material utilized for the shapedefining cavity or shell has low permeability and is rigid. It issupported in a cast low permeability material holding the forming cavityin two die members movable together by a hydraulic press. By making thistype of die member, induction heating along the tubular blank can bevaried so that subsequent cooling of specific portions of the structuralcomponent provides desired metallurgical characteristics. The endproduct does not need to have a uniform metallurgical characteristicassociated with the total processing operation which is the result ofthe Boeing process. Such process uniformly heats the sheet and does notquench harden the sheet.

In accordance with the present invention there is provided a method offorming an elongated tubular metal blank into a tubular structuralcomponent having a predetermined outer configuration, wherein the methoduses a shape imparting shell formed from a low permeability, rigidmaterial. The shell is in the form of a first and second half shell eachof which includes an inner surface defining the predetermined shape ofthe final structural component. The half shells have laterally spacededges which define a parting plane between the two half shells when thehalf shells are brought together. The half shells form a total shell orcavity having an inner surface defining the shape to be imparted to thestructural component as the blank is expanded into the cavity. One halfshell is mounted in one die member and the other half shell is mountedin the other die member so the die set can be opened and closed todefine the part forming cavity. By employing a rigid, hard materialdefining the shape to be imparted to the final part, the shell can besupported as a separate element in a cast non-magnetic material held inthe framework of the dies. By utilizing a cast material, together withan inner shell or cavity engaging the workpiece itself, the propertiesof the shell are not dictated by the compressive force carrying capacitynecessary for the cast material. Consequently, by using a cast materialwhich is different from the rigid, hard material actually engaging thetubular member during the forming process, both of these materials canbe optimized. Since the invention utilizes induction heating surroundingthe shell, the material of the shell and the material supporting theshell are both low permeability to be generally transparent to themagnetic fields created by the conductors embedded in the cast material.To expand the blank, the open ends of the tubular blank are pluggedwhile in one of the half shells in one of the die members. The otherhalf shell is then positioned over the blank and held in position bypressure in the general range of 50-500 tons. Thereafter, the tubularblank is formed into the final shape by inductively heating axialportions of the blank. When spaced portions are heated axially spacedconductors adjacent the shell or cavity are used. The heating is donewhile the tubular blank is forced into the cavity to create the desiredshape. Consequently, tube expansion is accompanied by forcing an inertgas, such as nitrogen or argon, at high pressure into the plugged blankuntil the blank conforms to at least a portion of the inner surface ofthe first and second half shells defining the shape cavity during and/orafter the tube is inductively heated. By utilizing conductors spacedaxially along the workpiece and embedded in the cast material around theshell, the metal of the blank is inductively heated to facilitate theforming operation caused by the expansion action of the internal gaspressure. By using the present invention, the total length of thetubular blank can be heated inductively and/or selected portions can beheated inductively. In practice, induction heating is normallyaccomplished to a greater extent where the primary formation orelongation is to be accomplished in practicing the invention. In thisaspect of the invention, there is provided a unique formation twocomponent die member. The inner component defines the shape and theouter component defines the compressive force absorbing mass. Thus, thetwo components of the member can be optimized. A better shape can beimparted to the workpiece and an inexpensive compressive force absorbingcast material can be used. This cast material is employed for embeddingthe induction heating conductors that inductively heat of the tubularblank during forming or prior to forming. Indeed, the induction heatingcan be before and/or during the gas forming operation.

In accordance with this aspect of the invention, the high hardness rigidmaterial or shell is ceramic and preferably fused silicon. It is alsopossible to select material from the class consisting of siliconnitride, silicon carbide, beryllium oxide, boron oxide and zirconium. Inthe preferred embodiment the shell has a thickness of ⅜-⅝ inches and iscast from silicon nitride with or without sintering. Then a hard cuttingtool type ceramic may be coated on the shaped surface. In anotherprocess for making the rigid hard shell, powder silica is compressed to50%-70% and then the shape is machined into the block. Vacuum removesthe air while nitrogen is used to penetrate. This gives a siliconnitride shell. It may be a block supported in the die member or a thinwalled shell. As can be seen, the ceramic material used to construct theshape imparting shell is different than the inexpensive ceramic materialforming the remainder of the die member, which material is merely acompressive force resistant material supported in a metal framework. Theshell may be coated.

It is possible to select materials, such as oxides, i.e. refractorycements, glass ceramics, high strength ceramics (e.g. silicon nitride,silicon carbide, zirconium oxide etc.). These materials can be eithermonolithic, or with various forms of reinforcements (composites), suchas ceramic particulate reinforced glass. As an example, in one processfor making the rigid hard shell, powder silica is merely compressed bymore than 60% of full density. In one process, a silica-based glassceramic is melted, mixed with silicon carbide reinforcement and castinto the desired shell shape.

In accordance with another aspect of the present invention, thepredetermined shape has an axial profile which may undulate. Thus, thefinal part may have curves and bends. It is within an aspect of theinvention to preform tubular blank into this axial profile so the blankgenerally corresponds to the profile of the final part.

In accordance with another aspect of the invention, the tubular blank isresistance heated by passing an alternating current, or direct current,through the sheet metal oft he blank preparatory to moving the hollow ortubular blank into the forming shell. Induction preheating is also used.Consequently, the total tubular blank is at an elevated temperature sothat the induction heating of the blank merely raises the temperaturebeyond the preheated temperature of the blank.

In accordance with another aspect of the present invention, theinduction heating is varied along the length of the tubular blank orover the locations of the flat hollow blanks whereby different locationsare inductively heated to different temperatures, at different timeintervals, to achieve optimal strain distribution control. Indeed, axialportions of the workpiece are inductively heated in different inductionheating cycles dictated by the desired metallurgical characteristics anddeformation amount at axial portions of the tubular blank. By changingthe induction heating effect along the blank preparatory to forming, orduring forming, the induction heating process is “tuned” withtemperatures at different locations on the tubular blank. In thismanner, the desired metallurgical characteristics and/or the optimumforming procedure is obtainable. The use of induction heating in thismanner to selectively process portions of the tubular sheet metal blankdistinguishes the present invention from any prior forming processes.

The use of induction heating to different degrees at various portions ofthe tubular blank allows thermal processing of the various portionsdifferently. Variations in the induction heating along the length of theblank can be accomplished by a number of coils or conductors along theforming cavity. The heating cycle of selected portions is controlled byvarying the frequency, the power, the distance of the conductors fromthe workpiece, the spacing between axially adjacent conductors and theinduction heating cycle time. By changing one or more of these inductionheating parameters, the tubular blank being formed has controlledheating along its length. The temperature is controlled. For steel, itis generally 1400° F. to 1800° F. Aluminum is heated to a lowertemperature. The objective is to produce a specific temperature thatcreates the proper formability plasticity.

In accordance with another aspect of the present invention, the heated,formed tubular structural component is transferred into a quenchstation. In the quench station, the previously inductively heatedstructural component is liquid and/or air quenched along its length. Inaccordance with another aspect of the invention, the quenching action isalso “tuned” along the length of the workpiece. By controlling theamount of heating during the forming process and the quenching time,flow rate and/or temperature of the liquid, metallurgical properties ofthe steel or aluminum forming the structural component is controlled atvarious portions along the length. By practicing the present invention,the tubular blank is inductively heated in a controlled fashion atvarious locations along the length of the blank. The resulting tubularstructural component is then quenched in a controlled fashion to dictatethe metallurgical characteristics along the various portions along thelength of the structural component.

In accordance with a more limited aspect of the present invention, asthe tubular blank is expanded into the shape of the shell or cavitycarried by the two spaced die members, portions of the tubular workpieceoutside of the die members is pushed into the cavity or shell to provideadditional metal to prevent drastic reduction in the wall thickness whensubstantial expansion of the tubular blank is dictated by the desiredfinal shape of the structural component. This procedure is a conceptused in hydroforming of steel tubular blanks. In accordance with still afurther aspect of the invention, the pressure of the forming inert gaswithin the tubular blank is sensed and controlled at the desiredpressure. The gas pressure is controlled in the general range of200-1000 psi which is sufficient to expand the inductively heatedworkpiece by using the method of the present invention. The gas pressureis controlled either by controlling the pressure introduced into theplugged tubular blank or, in the alternative, by venting the pressurefrom the blank.

In accordance with still a further aspect of the present invention,there is provided a die set for forming an elongated tubular steel blankinto a tubular structural component. This die set comprises a shapeimparting shell formed from a low permeability, rigid material. Theshell has a thickness of ⅜-{fraction (58/8)} inches and is preferablyformed from cast silicon nitride which is a hard cutting tool typeceramic. In practice, a non-sintered silicon nitride shell has a thincoating on the inner shaped surface of the shell formed by sputterdeposed dense silicon nitride. Coatings of silicon carbide and titaniumnitride have also been used. The hard shell is in the form of first andsecond half shells each of which includes an inner surface (preferably acoated surface) defining the predetermined shape, an outer support andmounting surface and spaced lateral edges which edges define the partingplane between the two half shells when the half shells are broughttogether. The halves form a total shell into which the tubular blank isexpanded. The first die member has an upper side and a lower side and anon-magnetic support framework for carrying the first half shell mountedin the metal framework by a cast compressive force transmittingnon-magnetic fill material. This fill material is preferably fusedsilica. The lateral spaced edges of the first half shell facingoutwardly from the lower side of the first die member. In a like manner,the second die member has an upper side and a lower side and anon-magnetic support frame for carrying the second half shell mounted inthe framework by a cast compression force transmitting non-magnetic fillmaterial. The fill material is preferably fused silica. The laterallyspaced edges of the second half shell facing outwardly from the upperside of the second die member. The first and second die members are Ifmoved together to capture the blank in the shape imparted shells formedby the hard, rigid shell halves. The two die members carry a shellformed from a hard, rigid material selected for the purposes of long diewear. The shell material is selected to maintain the desired shape ofthe shell for long periods of time. By using this type of die set, theinduction heating conductors or coils are embedded within the cast fillmaterial surrounding the shape imparting inner surface of the hard,rigid shell.

In accordance with another aspect of the present invention there isprovided a method of forming an elongated tubular blank into a tubularcomponent having a predetermined outer configuration. This methodcomprises plugging the open end or ends of the tubular blank, placingthe plugged blank into a shell or cavity with an inner surfacesurrounding the blank and having a predetermined outer configuration,forming the tubular blank into the component by inductively heatingalong the length of the blank by axially spaced conductors surroundingthe cavity, while forcing inert gas at a high pressure into the pluggedblank until the blank conforms to at least a portion of the innersurface of the cavity or shell to form the desired final component. Theinert gas is nitrogen or argon. The shell has a thickness in the generalrange of ⅜-⅝ inches and the metal being formed is steel and aluminum.

In accordance with still a further aspect of the invention, an elongatedtubular blank is formed into a tubular component having a predeterminedouter configuration. This method involves plugging the open ends of thetubular blank, placing the plugged blank into a shell or cavity with aninner surface surrounding the blank and having a predetermined shape,forming the tubular blank into the component by inductively heatingaxial portions along the length of the blank while forcing gas at highpressure into the blank until the blank conforms to at least a portionof the inner surface of a cavity and transferring the formed componentto a quench station where the component is selectively quenched alongits axial length.

The induction heating used in the present invention is varied alongaxial portions of the tubular blank and the quenching is also controlledalong the axial length of the blank. In this manner, the formingoperation is optimized and the metallurgical properties of the resultingstructural component are optimized. Since the invention is a hot formingprocess, it provides a means to significantly improve materialformability. Within the acceptable forming time (i.e. 15 seconds), ordeformable speed (strain rate greater than 0.1 per second), the processachieves more than 100% uniform tensile elongation for several aluminumalloys, as compared to about 30% in cold forming processes. The hotmetal gas forming provides enhanced formability, thus greatly enhancesmanufacturability of structural parts and offers increased designflexibility. Consequently, the process part has reduced weight, toolingcosts and development time.

The primary object of the present invention is the provision of a methodof forming a tubular metal blank into a tubular structural component,with the desired outer shape, which method controls the heating andmetallurgical characteristics by controlled induction heating andcontrolled quenching.

A further object of the present invention is the provision of a method,as defined above, which method overcomes the disadvantages ofhydroforming such as limited shapes, low die life and high equipmentcosts.

Still a further object of the present invention is the provision of amethod, as defined above, which method has reduced the tooling cost,reduced process cycle time, and increased design flexibility.

Yet another object of the present invention is the provision of amethod, as defined above, which method allows size or shape changessubstantially over 10% of the original tube diameter without requiringsecondary operations or material annealing operations betweenprocessing.

Still a further object of the present invention is the provision of adie set for practicing the method as defined above, which die setincludes a shell or cavity formed from a hard, rigid ceramic supportedby a non-magnetic cast fill material so the shell has long life and thefill material has high compressive force characteristics.

Another object of the present invention is the provision of a method, asdefined above, which method involves expanding a tubular workpieceheated inductively by controlled heating cycles. Then selectivelyquenching the workpiece is used to control the metallurgical propertiesof the finished product using rapid quenching, arrested cooling orcombinations thereof.

These and other objects and advantages will become apparent from thefollowing description taken together with the accompanying drawing.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a pictorial view of a representative tubular structuralcomponent formed by use of the present invention;

FIG. 2 is a side elevational view showing a machine for practicing thepresent invention;

FIG. 3 is a cross sectional view taken generally along line 3—3 of FIG.2;

FIG. 4 is a top view of a machine illustrated in FIG. 2;

FIG. 5 is a pictorial view of a multi-station platform for processingthe workpiece shown in FIG. 1 by using the present invention with anadditional processing step;

FIG. 6 is a cross sectional view taken generally along line 6—6 of FIG.5;

FIG. 7 is a pictorial view of sheet metal portions for making a complexH-shaped tubular blank to be formed by the method of the presentinvention;

FIG. 8 is a top plan view of the tubular blank using the plates of FIG.7 after the edges have been welded, but before the blank is trimmed;

FIG. 9 is a view similar to FIG. 8 with the tubular blank with the fourlegs trimmed to the desired length;

FIGS. 10 and 11 are pictorial views showing the operation of pluggingone of the open ends of a leg of the tubular blank shown in FIGS. 8 and9;

FIG. 12 is a pictorial view similar to FIG. 10 and 11 illustrating theplugged end of a tubular blank as it is being formed by air pressureintroduced through the plug;

FIG. 13 is a top plan view of the tubular blank shown in FIGS. 7-12 asit is being formed by pressurized gas while being selectively inductionheated;

FIG. 14 is a cross sectional view of the two die members used inpracticing the present invention with a differently shaped part wherethe induction heating coils or conductors are positioned along only oneside of the die member;

FIG. 14A is a cross sectional view of the two die members used inpracticing the present invention illustrating the use of a connector forjoining the conductors, shown as solid lines, in the induction heatingmechanism of the invention;

FIG. 14B is a cross sectional view illustrating induction heating of aselected area of the tubular blank as it is being formed in the diemembers;

FIG. 14C is a schematic view of a flux yoke to selectively increase theinduction heating in specific areas along the tubular blank as the blankis being formed;

FIG. 14D is a schematic view illustrating the use of a Faraday shieldshiftable along certain areas of the induction heating conductors toalter the heat profile along the length of a blank being formed;

FIG. 15 is a cross sectional view of the two die members used inpracticing the present invention for producing a particularly tubularstructural component with a different expanded shape and illustratingthe distribution of induction heating coils along the length of thecavity for forming the tubular blank;

FIG. 15A is a schematic block diagram showing power supplies to developthe induction heating parameters used in the conductors or heating coilsshown in FIG. 15;

FIG. 16 is a schematic cross sectional view of a die member for forminga tubular blank having a undulating profile wherein selective inductionheating coils or conductors are positioned at different areas in the diemember to inductively heat the tubular workpiece during the formingoperation using different induction heating cycles;

FIG. 17 is a pictorial view of a closed die set for use in practicingthe present invention, wherein the coils or conductors along the lengthof the die set are connected in series in each of the die members;

FIG. 18 is a pictorial view, similar to FIG. 17, wherein the conductoror coils are connected in series from one die member to the other. Thisrequires flexible connectors or other movable connectors to allowseparation of the die members for loading and unloading the tubularblank;

FIG. 19 is a schematic view of the tubular structural component after ithas been formed and inductively heated along its length with selectedquenching stages illustrated;

FIG. 20 is a side elevational view illustrating an aspect of the machinefor in-feeding a metal as the tubular blank is being formed into thetubular structural component;

FIG. 21 is a view similar to FIG. 20 showing control elements in blockdiagram form as used in a control system of the preferred embodiment ofthe present invention;

FIG. 22 is a pictorial view showing the preform die used in thepreferred embodiment of the present invention with a curved workpiece;

FIG. 23 is a pictorial view of the lower die member used to form acurved workpiece preformed by the preform die in FIG. 22;

FIG. 24 is a partial pictorial view illustrating the end portion of thelower die member used in the preferred embodiment of the presentinvention;

FIG. 25 is a pictorial view of the end portion of the quench station forselectively quenching previously inductively heated portions of thefinal tubular structural component;

FIG. 26 is a pictorial view showing the quench station used in thepreferred embodiment of the present invention;

FIG. 27 is a cross sectional view showing two induction heating coilsaround the forming shell or cavity with the coils separated to providedistinct induction heating cycles during the forming of the tubularblank;

FIGS. 28A and 28B are views similar to FIG. 27 illustrating operatingcharacteristics of the selectively controlled induction heating duringthe forming of the tubular blank; and,

FIG. 29 is an end view of a cooling mechanism for causing arrestedcooling of the heated workpiece after it has been formed.

PREFERRED EMBODIMENTS

Referring now to the drawings wherein the showings are for the purposeof illustrating the preferred embodiments only and not for the purposeof limiting same, FIG. 1 shows a finished tubular structural component Aformed by using the preferred embodiment of the present invention asschematically illustrated as machine 20 in FIGS. 2-4. This part isillustrated as a quite simple shape for ease of discussion. Thepreferred shape is the workpiece processed by the apparatus shown inFIGS. 22-26. However the disclosure associated with the simple shape ofcomponent A applies to all shapes. Machine 20 includes an inlet station22 for preprocessing a plugged tubular blank which will be describedlater. This preforming operation may involve bending the tubular blankaxially into a preselected general contour or profile as shown in FIG.22 for the preferred workpiece formed by the invention. Thepreprocessing of tubular workpiece or blank may involve preforming orheating. Preheating is used in one embodiment of the invention byresistance heating the total blank or workpiece at input station 22.Resistance heating of the blank preparatory to forming by hot gas inaccordance with the invention is performed by directing, preferably, analternating current through the tubular blank or workpiece as it ispositioned at input station 22. Resistance preheat can be direct 60cycle heating. Also, induction resistance heating may be used to changethe thermal profile during the preheat step. The frequency can be 60cycle or higher. It is possible to provide a combination of direct andinduction preheat. For illustration purposes, FIG. 2 shows the tubularplugged blank a in station 22, which station can be considered merely aloading station if preforming and/or preheating is not required. Insummary, input station 22 is used for preforming, preheating or merelyloading. The preforming operation and the preheating operation reducesthe amount of time and energy needed to form workpiece or blank a intostructural component A at the processing station 24. This stationperforms the essence of the invention wherein a plugged workpiece ortubular blank a is conductively heated by a plurality of coils orconductors spaced along workpiece a at station 24 while a high pressureinert gas, such as nitrogen or argon, is directed into the tubularworkpiece a for expanding the workpiece into a cavity surrounding theworkpiece at station 24. After the workpiece a has been inductivelyheated and formed by inert gas into the desired structural configurationshown in FIG. 1, it is transferred into quench station 26 where a quenchliquid, or air is directed toward the outer surface of the heated andformed structural component to cool the component at a rate determiningthe necessary metallurgical properties of the finished product. Insummary, the invention is the expansion of a tubular plugged workpiece ainto the desired shape shown in FIG. 1 by inductively heating theworkpiece along its length while expanding the workpiece into apredetermined shape determined by a die cavity with inert gas and thenimmediately moving the hot formed workpiece into a quenching stationwhere a quenching operation creates the desired metallurgical physicalproperties. By rapid quenching, a hardened structure of the workpiece iscreated. Slow quenching by liquid or air could be used to temper certainportions along the length of the finished component A. Thus, byinductively heating and selectively quenching the hot metal gas formedstructural component, the shape of the component is obtained at the sametime metallurgical properties along the length of the structuralcomponent are also obtained. This is a novel and heretofore unobtainableresult for a metal tubular structural component. The blank when formedof steel has a wall thickness of 0.40-0.35 inches and is preferably lessthan 0.20 inches. The steel is a single or dual phase, high strengthsteel. When aluminum is used for the metal formed, 5083 aluminum andseveral other 5000 series aluminum alloys have been used with a wallthickness of 3 mm.

Although a number of machines and mechanical components could be usedfor practicing the present invention, the preferred embodiment involvesa multi-station machine 20 shown in FIGS. 2-4 having the loading orpreprocessing station 22, the actual hot metal gas forming station 24and the novel quench station 26. In the illustrated machine 20, there isa lower support frame 30 having an upper fixed table 32 overlaid by anupper fixed head 34. Transfer mechanism 40, shown in phantom lines, is awalking beam type of transfer mechanism for shifting the plugged blank ainto station 22 for moving the blank or workpiece a to station 24 whereit is hot metal gas formed in accordance with the invention and for thenmoving the formed structural element A to quench station 26 where theheated and formed workpiece is quenched along its length by liquidand/or air quenching. Referring now to initial or loading station 22, agenerally rectangular holder 50 has a nest 52 for receiving the pluggedtubular blank or workpiece a. The optional preforming shown in FIG. 22or resistance heating is not illustrated. From loading station 22,workpiece a is moved to the hot metal gas forming station which involvesa die set 60 having a lower die member 62 and an upper die member 64which are brought together to form a cavity or shell 66 defining thedesired outer configuration of structural component A after it has beenprocessed in accordance with the present invention. Lower die member 62is supported on fixed table 32, whereas the upper die member is carriedby a platen 70 movable on rods or posts 72 by four spaced bearinghousings 74 between a closed lower position shown in the solid lines ofFIG. 2 and an upper open position shown by the phantom lines in FIG. 2.Post 72 not only reciprocally mounts the upper die member 64, but alsofixes machine head 34 with respect to the lower fixed machine table 32.Movement of die member 64 is accomplished by cylinder 80 fixed on head34 and joined to platen 70 by rod 82. Movement of the rod 82 by cylinder80 raises and lowers die member 64 to open and close the die member 60for loading and unloading station 24. As will be described later, one orboth die members include a number of axially spaced induction heatingconductors embedded within the die members to heat the metal of blank ato a temperature about 1800° F. The temperature can be varied along thelength of the workpiece. Such heating is done by induction heating whichraises the temperature of the workpiece by inducing voltagedifferentials using an alternating current in the coils or conductorssurrounding the workpiece during the forming operation. In the preferredembodiment, collets 104, 106 surround the ends 10, 12 which extendoutwardly from holes 68 in die set 60 as best shown in FIGS. 3, 4, 17and 18. These collets are forced inwardly by feed cylinders 100, 102,respectively, so that metal is fed into cavity of shell 66 during thehot metal gas forming process in a manner similar to such in-feed ofmetal during hydroforming of steel. Inert gas, nitrogen or argon, athigh pressure in the range of 200-1000 psi is forced into the heatedworkpiece to expand the workpiece into shell or cavity 66. The gas iscapable of expanding the steel which has a wall thickness in the rangeof 0.04-0.35 inches and preferably less than 0.25 inches. The metal isheated to a temperature in the general neighborhood of 1800° F. andsubjected to an inert gas pressure of 200-1000 psi. This forming processnormally takes less than about 20 seconds and preferably about 10seconds. In practice, the hydraulic pressure from cylinder 80 exerts acompressive force between die members 62, 64 which is about 100 tons.With this high holding force on die set 60, the hot metal gas formingprocess does not separate die members 62, 64 during the formingoperation. When the hot metal has been formed in station 24, cylinder 80moves upper die member 64 by moving platen 70 upward. After the die hasbeen opened, the formed structural element A is moved by transfermechanism 40 from station 24 to station 26 best shown in FIGS. 2 and 4.

A modification of machine 20 is illustrated in FIG. 5 wherein fourstations are employed on platform or table 32 a. In this modification, apreformed station 22 a, such as shown in FIG. 22, is provided with anest 52 a. Nest 52 is used for resistance heating. At station 24, theshape defining shell or cavity 200 of the lower die member 62 isillustrated along with induction heating coils or conductors C. In usingthis modified machine, workpiece a is placed in nest 52 a and shapedinto the desired profile. Thereafter, walking beam transfer mechanism 40shifts the workpiece nest 52 where it is subjected to resistanceheating, preferably with A.C. current. The workpiece is then transferredto shell or cavity 200 of die member 62. The upper die member is thenclosed and the workpiece is hot metal gas formed. The hot formedstructural component is then moved to station 26 and quenched aspreviously described.

Details of die set 60 are shown in FIG. 6 wherein die set 62, 64 includean inner shell or cavity 200 having half shells 200 a, 200 b,respectively. The shells are formed from a low permeability, rigidceramic material having a high hardness. In practice, the material isfused silica; however, the material could be selected from the classconsisting of silicon nitride, silicon carbide, beryllium oxide, boronoxide and xirconia. In the preferred embodiment, a silicon nitride shellwith a wall thickness of ⅜-⅝ is formed with the desired inner surfaceshape and a coating of dense ceramic layer placed on the surface bysputtering or chemical vapor deposition. Thus, a dense ceramic layer isapplied to a non-sintered silicon nitride shell. In an alternative,powdered silica is compressed to about 50%-70% and machined to thedesired shape. Then the block is vacuum a exhausted while nitrogen isimpregnated into the shell. As an aspect of the invention, the lowpermeability rigid material forming shell or cavity 200 having thedesired contour and shape for the finished structural component isselected for its wear resistance and maintenance of the desired shapewithout deterioration over many forming cycles. In the past, a hard,rigid shell was not used for creating the forming cavity between diemember 62, 64. By using a separate rigid shell for the cavity in the dieset, a less expensive and compressive force resisting fill material 210can be selected for the body portion of die members 62, 64. Fillmaterial 210 is non-magnetic and compression resistant. Fused silica oreven cement has been used successfully since shell 200 is the precisioncomponent. Fill material 210 is selected for its pressure resistance andits ability to maintain shell 200 rigidified. Ceramic fill material 210is selected for its compression resistance characteristics and is acastable ceramic having strength and hardness substantially less thanthe rigid ceramic shell 200. In practice, any of a number of castableceramics, such as fused silica, or cement is employed for the support ofrigid, hard shell 200. Die members 62, 64 are held together with aframework 212, 214 which is a non-magnetic material, such as aluminum orstainless steel. The 100 tons of pressure is applied between castableceramic material 210 of die members 62, 64 for holding rigid, hard shellor cavity 200 in place during the forming process in station 24.

Ceramic fill material 210 encapsulates and supports the number ofaxially spaced conductors C forming the induction heating mechanism ofdie set 60. In the preferred embodiment, as shown in FIG. 6, conductorsC include arcuate portions 220, 222 conforming to the outerconfiguration of shell 200. Conductors or coils C are connected inseries, as shown by connector 224 and are powered by an alternatingcurrent power source 230 which, in practice, operates at a frequencygreater than about 3 kHz and preferably greater than about 10 kHz.Axially spaced conductors C are joined by connectors 224 to place themin series with the power supply 230 in accordance with standardinduction heating practice. Encircling coils in shell 200 are formed byjoining upper and lower conductors C, as shown in FIG. 6. Variousarrangements can be used for connecting the set of conductors C in diemember 62 and die member 64. The conductors extend across the dies andare connected in a series circuit with a power supply, such as powersupply 230. This power supply is an inverter in practice. Die set 60 isopened and workpiece a is placed in the cavity defined by shell 200.Then the die set is closed to combine workpiece a in cavity or shell 200where it is heated inductively along its length and formed byintroducing hot inert gas. In practice the conductors for the inductionheating of the workpiece are non-magnetic, high resistivity steel(Inconel) tubes with water cooling. These conductors have greaterstrength and are better suited modules than copper tubes.

The present invention can be used for producing a large variety ofstructural components. To illustrate the versatility of the presentinvention, an H-shaped structural element B is formed by the method ofthe present invention. Tubular blank b is shown in FIGS. 7-12. TwoH-shaped steel plates 250 a, 250 b with a laser welded center portion250 c are joined together in a manner where legs 252 a, 254 a, 256 a,258 a are seam welded to legs 252 b, 254 b, 256 b and 258 b,respectively to form tubular blanks identified as legs 252, 254, 256 and258 in FIG. 8. The outer edges of the plates are laser welded togetheras shown at seam W in FIG. 10. Overlying welded legs 252 and 254 form asingle hollow workpiece. In a like manner, seam legs 256, 258 form asingle hollow workpiece. These tubular legs are like workpiece a shownin FIGS. 2 and 4. Center portion 250 c is welded together to form agenerally flat structural element, but it does not constitutenecessarily a portion of the tubular workpiece to be formed. After seamwelding legs 252, 254, 256 and 258 to form workpiece b, the legs aretrimmed to the desired length by removing excess portions 262, 264, 266and 268 by trimming the ends of the respective legs. This trimmingaction produces a workpiece b, as shown in FIG. 9, which workpiece is inthe form of two generally parallel tubular blanks. In accordance withthe invention, plug 270, having a wedge shaped nose 272, is forcedhydraulically into the end of each of the legs 252, 254, 256 and 258.Each of the plugs 270 includes a gas inlet 274 with a flared gas passage276. As shown in FIGS. 10-12, plugs 270 are forced in the end of each ofthe legs so gas G can be forced into each of the legs to expand the legsinto the shape of the H-shaped shell of die members 60, 62 having shellsor cavities formed in accordance with the desired shape of structuralcomponent B illustrated in FIG. 13. During the forming process, theworkpiece is heated inductively by coil 280 encircling legs 252, 256 anddriven by high frequency power supply 282. In a like manner, inductionheating coil 290 encircles legs 254, 258 and is energized by a highfrequency power supply 292. In accordance with an aspect of theinvention, the coils 280, 290 are operated at different cycles 50 of therespective legs being formed are heated differently, in accordance withan aspect of the process of the invention. Thus, portions 300, 302 oflegs 252, 256, respectively, are heated substantially less than portions304 and 306 of legs 254, 258. This representation of the presentinvention illustrates that the induction heating equipment associatedwith the die set allows processing of the workpiece being formed atdifferent temperatures to obtain the desired forming rate. It is part ofthe invention that a greater portion of legs 254, 258 be heated duringthe forming process than the portion being heated in legs 252, 256.However, all of the metal being formed must be at a temperature of atleast about 1400-1500° F. This is a novel concept of heating portions ofthe workpiece differently. In the past, when induction heating was usedfor superplastic deformation of sheet material, the total sheet materialwas heated the same. Thus, the requirement for different heating atdifferent sections could not be accommodated by use of the priorsuperplastic heating processes used for flat plate material.

A primary aspect of the invention is the ability of the inductionheating equipment associated with the die set 60 to selectively heatdifferently different portions of the tubular blank or workpiece beingformed by high pressure gas. As mentioned above, this ability to “tune”the induction heating along various sections of the workpiece beingformed is novel and has not been done previously. Variations in theinduction heating of the workpiece being formed by high pressure gas inaccordance with the invention can be accomplished by using variousinduction heating arrangements. One of these arrangements is illustratedin FIG. 14. The cross sectional shape of the forming shell includes adome portion 310 in upper die member 64 and a generally flat portion 312in lower die member 62. It is desired to heat the portion of theworkpiece being formed greater adjacent the dome shaped portion 310.Consequently, axially spaced conductors 320 with water passage 322 are118 spaced along the dome portion of the shell in upper die member 64.These conductors 320, several of which are aligned along the axis of theworkpiece, each have an arcuate segment 330 with straight legs 332, 334.There are no conductors adjacent flat portion 312 in lower die member62. By using this configuration, induction heating is accomplished atthe top side of the workpiece that is going to have the most movement ofmetal during the forming process. A generally circular workpiece a isplaced between shell potions 310, 312 and is expanded by gas as it isbeing heated by induction heating on the side adjacent the dome portionthrough the induction heating effect of the arcuate segments 330 ofaxially spaced conductors 320. This implementation of the presentinvention shows how the heating can be accomplished along the length ofthe workpiece at different heating cycles or different magnitudes. Thiscan be done by encircling conductors such as conductors 340, 342 placedin series by connector 344 as shown in FIG. 14A, by the arrangementshown in FIG. 14 or by the selective heating arrangement illustrated inFIG. 14B.

In FIG. 14B, a generally rectangular tubular workpiece d is to be formedin half shells 350, 352 which forms an encircling configuration when dieset 60 is closed. In this implementation of the present invention, comer360 of workpiece d is to be heated during the forming process. This isaccomplished by conductors 370, 372 at the opposite ends of fluxconcentrator 374 formed of a high permeability material, such asFerrocon. As shown in FIGS. 14, 14A and 14B, induction heating ofselected portions along the length of the workpiece being formed by highpressure gas is used to control the forming process. This is alsoemployed for the purposes of controlling the metallurgical properties ofthe final product, as will be explained later. By changing theconductors 340, 342 along the length of the workpiece being formed, asshown in FIG. 14A, a different amount of heating can be accomplishedalong the length of the workpiece or on one side of the workpiece.Another arrangement for changing the heating effect along the length ofthe workpiece is illustrated in FIG. 14C wherein the axially spacedconductors 340 are joined in series with conductors 342 by connectors344 as previously described. In one or both of the die members, there isprovided a flux yoke 380 formed of high permeability material which islocated along the axial length of the workpiece to shunt the inductionheating effect of the coils 340, 342. In this manner, throughout thelength of the workpiece, a constant encircling coil for inductionheating is provided. This is the preferred arrangement. To change theamount of heating caused by this continuous encircling coil, the die setis provided with a flux yoke 380 positioned axially along the workpiece.This changes the heating effect at various axial positions along theworkpiece without really changing the induction heating coilarrangement. Another system for changing the induction heating isillustrated in FIG. 14D where Faraday shield 390, including a capacitor392 and an adjusting resistor 394, is provided at various locationsalong the length of the workpiece. The effect of the Faraday shield isadjusted at various positions to decrease the amount of inductionheating caused by certain portions of the coil encircling the workpiece,as schematically illustrated in FIGS. 14A, 14C. As illustrated in thesefigures, a variety of electrical options are available to change theamount of heating along the length of the workpiece or at differentsections of the workpiece while the workpiece is being expanded by gasin accordance with the invention. The coils or conductors C are spacedabove shell 200 and the heating effect is changed to control the amountof, and location of, different heating effects.

The versatility of tuning the induction heating along the length of theworkpiece is illustrated in another embodiment of the invention whereina tubular workpiece is to be formed into a complex tubular structuralshape as defined by shell 200′ in die members 62′, 64′ of die set 60′ asshown in FIG. 15. This shell will cause the tubular workpiece to havedifferent diameters and shapes in areas 402, 404, 406, 408 and 410. Inthese different areas, a different amount of heat is required fordeformation and the desired characteristics of the workpiece.Consequently, the die members are provided with a plurality ofencircling induction heating coils 402 a, 404 a, 406 a, 408 a and 410 a,respectively. These encircling coils are spaced axially along the shellor cavity 400 defining the final outer shape of the tubular structuralcomponent being formed by using the present invention. In accordancewith this aspect of the invention, each of the separate coils has aspecific frequency and a specific power level. Several power suppliesPS1, PS2, PS3, and PS4 are provided to create the different frequenciesand power levels for coils 402 a-410 a. As illustrated, power supply PS1has a frequency F1 and a power level P1. This power supply is connectedto encircling inductors 402 a and 408 a. In the same fashion, powersupply PS2 has a frequency F1 which is the same as PS1 but a differentpower level P2. This power supply energizes encircling coil 410 a. In alike manner, power supply PS3 has a frequency of F2 and a power level ofP3. This power supply drives encircling inductor 404 a. In a likemanner, power supply PS4 has a frequency of F3 and a power level P4 forenergizing encircling coil 406 a. By changing the heating frequency andpower level the heating cycle during the forming process is modulatedand changed along the length of the workpiece. This is used not only forcontrolling the amount of heat for the purposes of optimizing theforming operation, but also to optimize the metallurgical processing ofdifferent sections of the workpiece. It is necessary to raise thetemperature of the total length of the workpiece being formed to atemperature in the range of 1400° F.-1800° F. Consequently, the areas ofshell 200′ without coils or conductors will be short if they exist atall. It is preferred to use a large number of conductors with theheating effect changed, such as shown in FIG. 15 but by variousarrangements.

Another feature employed in an alternative of the present invention isillustrated in FIG. 16 wherein shell 420 has a modified profile, but auniform cross section, In this embodiment of the invention, an inductionheating coil is provided around the total length of the workpiece beingformed. This is the preferred arrangement as opposed to the embodimentof the invention shown in FIG. 15 wherein selective areas of theworkpiece are provided with encircling inductors. Where all areas haveencircling inductors, the heating along the length of the workpiece isaccomplished by using different power supplies as shown in FIG. 15A.Different regions of the workpiece can be heated sequentially, or withadjustable heating power, to achieve desired strain distribution.However, as shown in FIG. 15, it is also possible to not energize aportion of the encircling inductors or energize a portion for a shortertime at a lower power. The shell 420 is divided into sections 422, 424,426, 428 and 430. Between sections 426 and 428 there are encirclinginductors that could be used for induction heating; however, inaccordance with an aspect of the invention, these induction heatingcoils are not subject to being energized. Thus they do not causeinduction heating, even though they are present. Such uniformdistribution of the induction heating coils as used in the preferredembodiment is illustrated in FIGS. 17 and 18. Conductors C are connectedin series by connectors 450 and powered by separate power supplies PS5for upper die member 64 and PS6 for lower die member 62. In FIG. 18,flexible connectors 460 are between the upper and lower die member in asingle power supply PS7 is used. In FIG. 18, connectors 460 are flexibleto allow for opening and closing of the die set for loading andunloading the workpiece. Opening 68 at the end of the die setaccommodates protruding ends 10, 12 of the workpiece as schematicallyillustrated in FIG. 1. These ends are necessary for plugs to introducethe high pressure inert gas.

Another aspect of the present invention is controlled cooling after hotforming at station 26. The controlled cooling process is either aquenching operation, or an operation cooling the workpiece at a reducedrate, depending on the metallurgical characteristics of the workpiecematerial and the performance requirements of the final structure. Theuse of the terminology of “quench” is to represent the general on-lineheat treating process and to explain the capability of the new formingprocess for optimizing the material performance. This feature isschematically illustrated in FIG. 19 wherein a finish hot formed tubularworkpiece is positioned in the quench station 26. Along the length ofthe workpiece different quenching orifices are used. This is illustratedas quench station 500, 502, 504 and 506, each of which is individuallycontrolled in either liquid or gas quenching. By using a precisequenching cycle with a specific heating cycle during the processing ofthe workpiece D, the metallurgical properties of the finished productare controlled. The modulation of induction heating along the length ofthe workpiece, in combination with the precise control of the quenchingalong the workpiece, creates an improved finished product wherein themetallurgical properties along the workpiece are optimized based uponthe desired amount of heating, the temperature of the heating cycle andthe quenching cycle. This is a further aspect of the present inventionand is completely different than procedures heretofore used in gasforming of metal sheets. It is preferred to use steel in the invention,since steel has the capability of modified metallurgical propertiesalong its length.

The cooling or quench station 26 sometimes uses distortion controllingrestraints to give size control. When cooling aluminum a high rate ofuniform cooling, as by sprays, is used with the mechanical restraints.

The invention uses the concept of positively feeding metal into thecavity of the die set as the metal is formed. This concept isschematically illustrated in FIG. 20 wherein a function generator 510controls servo cylinder 100 forcing the collet 104 inward slightlyduring the hot metal gas forming process. The process is started asindicated by block 512. In a like manner, cylinder 102 is moved inwardlyby a signal from error amplifier 520 having a sensed force signal inline 524. The level of the actual force applied by cylinder 102 iscompared to the level of a reference signal in line 522. The errorsignal controls servo cylinder 102. The illustration in FIG. 20 isrepresentative. This concept is also used in hydroforming and will beused in practicing the present invention when further implementation ofthe invention is made. In accordance with an aspect of the inventionschematically represented in FIG. 21, plugs 270 have gas inlets oroutlets 274. Gas supply 550 provides an inert gas such as argon at apressure between 200-1000 psi. This gas is directed to workpiece B by aninlet valve 552. An exhaust valve 554 allows decrease in the internalpressure of workpiece B. Valve 552 increases the gas pressure whileexhaust valve 554 decreases the pressure. These valves are controlled byan error amplifier 560 having an outlet 560 a that operates valve 552.In the alternative, line 560 b controls exhaust valve 554. Functiongenerator 562 provides one input 562 a to error amplifier 560. The otherinput 570 a is created by pressure sensor 570 within workpiece B.Pressure sensor 570 provides a signal in lines 570 a that is comparedwith the output of function generator 562 at line 562 a. This determineswhether, at a given temperature, represented by the signal in line 572 afrom sensor 572 additional pressure or less pressure should be providedin workpiece B. Consequently, the pressure is maintained at the desiredselected level associated with a given temperature. Controlarrangements, both analog and digital, can be used in the preferredembodiment of the present invention.

The invention has been described with a simple shaped workpiece. In thepreferred embodiment, the cylindrical workpiece is to be formed into atubular structural component having an undulating profile in the axialdirection. Thus, a preform step is needed to prepare the workpiece. Thispreform step is followed by a preheat and then hot metal gas forming instation 24. Consequently, a preform die 600, as shown in FIG. 22, ismounted by base 602 at station 22 of machine 20 as shown in FIGS. 2-4.This die has an elongated nest 610 with the desired profile to beimparted to the cylindrical workpiece preparatory to the formingoperation. In this manner, the cylindrical sheet metal workpiece, whichhas been plugged, is preformed in nest 610. This forms the cylindricalworkpiece so it will easily fit in the cavity of die set 60 for thesubsequent forming operation. FIG. 23 illustrates lower die member 700for the workpiece preformed by the die 600 in FIG. 22. This lower diemember is matched with a similar upper die member for the gas formingoperation. It includes shell 702, framework 704 and a large number ofaxially spaced conductors 710. These axially spaced conductors of theinduction heating equipment are embedded within the ceramic fillmaterial 720 of lower die 700. Conductors C are spaced along the shell asmall distance less than 0.50 inches. FIG. 24 is a pictorial enlargedview of one end of lower die member 700 as shown in FIG. 23 with a shell712 and opening 714. Fill material 720 is removed to illustrate theencircling, closely spaced conductors 710 supported in framework 704.For the preferred preformed workpiece processed by the die set shown inFIG. 22 and the lower die member shown in FIGS. 23 and 24, there isprovided a quench unit 750 mounted at station 26 of machine 20. Thisquench unit is illustrated in FIGS. 5 and 26 as including a lowersupport base 752 having upstanding quench stands 760 and support stands760 a which may not be used for quenching. In quench stands 760, theheated formed workpiece is supported by nest 762 having quenching holes764 directing quench liquid onto the heated workpiece from inlets 766. Acover 770 shown in FIG. 26 is positioned over base 752 during thequenching operation to allow proper quenching of the workpiece. Opening772 provides clearance for quench inlets 766. Nest 762 a in stands 760 amerely support the heated workpiece during the quenching operation.However, they can be used for quenching of this area of the workpiece ifneeded. Quench stands 760 receive the desired amount of quenching liquidfor the quench operation as discussed in connection with FIG. 19. Byusing selective quenching, together with selective heating, the formingoperation is optimized. In addition, the metallurgical properties of thefinal formed structural component are optimized. In accordance with theinvention, coils or conductors are closely spaced along the die membersand quench stands are also closely spaced along quench unit 750.However, the amount of heating and the amount of quenching is controlledto give effective forming and desired properties of the finishedproduct.

A further feature of the present invention is illustrated in FIGS. 27,28A and 28B wherein a central multi-turn induction heating coil 780surrounds the cavity into which the hollow workpiece illustrated as asingle sheet E is to be formed by gas. A second induction heating coil782 includes spaced sections 782 a, 782 b on opposite ends of centralcoil 780. A profile formed by coil sections 782 a, 782 b with coil 780is the shape of the cavity 200 into which workpiece E is to be formed.Since coils 782 a, 782 b are close to workpiece E before it is formed,they heat the axially spaced sections X before the center portion Y ofthe workpiece is heated. Thus, the forming operation first causesmovement of sheet E in area X, as shown in FIG. 28B. Thus, during theinitial heating of the workpiece, which is a tube, the tube deformsfirst in areas adjacent the closer induction heating coil section 782 a,782 b. If the heating operation were discontinued at that time, theinvention would still have been performed in that the portions X wereformed into the shape of the cavity 200. With continued heating and gaspressure, workpiece E eventually shifts into the full cavity 200,defined by the contour of the coils 780, 782, as shown in FIGS. 27, 28Aand 28B. These schematic representations are used to illustrate that theinduction heating affects the ease of forming the workpiece during thehot metal gas forming process. The closer the coils are to the metalconstituting the workpiece E, the greater the heating effect. However,the heating equalizes as the workpiece assumes the final shape of theshell 200.

By providing controllable pressures for the inert gases, selectivelocation or operation of the induction heating conductors along and atvarious positions around the shell, and selective, controlled quenchingthe forming process is controlled to avoid a necking and/or wrinklecondition. Coordination of these acts with controlled in-feeding ofmetal produces uniform end products. During the process, the inductionheating at certain areas can be performed in die set 60 before finalheating and forming. During the forming, the gas pressure can bemodified, and in some examples is modified together with the inductionheating being modified on a time basis. By selective heating andmodified heating during the forming process the flow of metal iscontrolled. This is thermal enhanced intelligent forming. The inventionis not restricted to heating of a workpiece to a given amount during gasforming at a fixed pressure.

The workpiece being formed by the invention is a hollow structure orblank formed from a thin (0.40-0.35 inches) electrically conductivematerial, preferably steel (for hardening) and aluminum. However, brassand titanium have been successfully formed. After the metal has beeninductively heated by cycles where areas are heated selectively, atdifferent times and different temperatures, the workpiece is selectivelyquenched at station 26 by liquid or air at controlled times and cycles.This quenching operation gives aluminum dimensional stability. Thequenching operation is by a rapid quench cycle with liquid or gas or anarrested cooling quench as disclosed in U.S. Pat. No. 4,637,844,incorporated by reference herein. Combinations of rapid quenching andarrested cooling can be used at different portions of the inductivelyheated and formed workpiece. It has been found that some steels used forthe automobile industry should be cooled at a slower rate to maintaintheir high strength whereas other steels are quenched to be hardenedafter heated for forming. Mist cooling, arrested cooling, and rapidquenching are selectively used to obtain the desired final metallurgicalproperties in all areas of the final product. This procedure is alsoused for various aluminum alloys formed in accordance with theinvention.

In some processes, arrested cooling is used wherein the workpiece isquenched to a given temperature and held at that temperature for aselected time. Such procedure is illustrated in FIG. 29 whereinworkpiece 800 is surrounded by hot fluid manifolds 810 and 812 fordirecting fluid at a given temperature above ambient from nozzles 810 a,812 a (only a few of which are shown). This action cools workpiece 800to the temperature of the hot fluid where it is held until the fluidflow is stopped. This process can be used to obtain banite or to obtainother processing objectives.

The invention has been described in connection with either the preferredpreformed workpiece or a non-preformed workpiece with a simple shape.The shape of the workpiece is not important. The various disclosedapparatus can be used interchangeably to form the desired hot metal gasformed hollow structural component of various workpiece shapes. Theprocess involves a tubular metal workpiece which is plugged and subjectto high gas pressure in the range of 200-1000 psi. During this process,the metal is heated by induction heating. In accordance with an aspectof the invention, the heating process is modulated along the length toaccomplish the desired forming operation and desired heat distributionduring the forming process. In accordance with a novel aspect, theheated workpiece is then quenched selectively along its length to createthe desired metallurgical properties of the finished product. Theinduction heating while forming by inert gas followed by quenching ofthe final part is a novel method and obtains desired metallurgicalproperties. Other modifications can be made in the present inventionwithout departing from the intended spirit and scope as defined in theaccompanying claims.

Having thus defined the invention, the following is claimed:
 1. A methodof forming an elongated tubular metal blank with first and second ends,at least one of said ends being open, into a tubular structuralcomponent having a predetermined outer configuration, said methodcomprising: (a) providing a shape imparting shell formed from a lowpermeability, rigid material, said shell being in the form of first anda second half shell, each of which includes an inner surface definingsaid predetermined shape, an outer support and mounting surface andspaced lateral edges which edges define a parting plane between said twohalf shells when said half shells are brought together to form saidshell; (b) providing a first die member with an upper side and a lowerside and having a non-magnetic support framework for carrying said firsthalf shell mounted in said framework by a cast compression forcetransmitting non-magnetic material with said laterally spaced edges ofsaid first half shell facing outwardly from said lower side of saidfirst die member, said cast material having a strength and hardnesssubstantially less than said rigid material; (c) providing a second diemember with an upper side and a lower side and having a non-magneticsupport framework for carrying said second half shell mounted in saidframework by a cast compression force transmitting non-magnetic materialwith said laterally spaced edges of said second half shell facingoutwardly from said upper side of said second die member, said castmaterial having a strength and hardness substantially less than saidrigid material; (d) plugging said open end or ends of said tubular metalblank; (e) placing said plugged tubular metal blank into said secondhalf shell in said second die member; (f) moving said first die memberrelative to said first die member to capture said tubular metal blank insaid shape imparting shell; (g) forming said tubular metal blank intosaid component by inductively heating axial portions of said tubularmetal blank by axially spaced conductors adjacent said shell while orbefore forcing gas at a high pressure into said plugged tubular metalblank until said tubular metal blank conforms to at least a portion ofthe inner surfaces of said first and second half shells to form saidcomponent.
 2. The method as defined in claim 1, wherein said rigidmaterial is ceramic having a high hardness.
 3. The method as defined inclaim 2, wherein said cast material is castable ceramic.
 4. The methodas defined in claim 1, wherein said rigid material is fused silicaimpregnated with nitrogen.
 5. The method as defined in claim 1, whereinsaid rigid material is selected from the class consisting of siliconnitride, silicon carbide, beryllium oxide, boron oxide, and zirconia. 6.The method as defined in claim 5, wherein said cast material is castableceramic.
 7. The method as defined in claim 1, wherein said cast materialis castable ceramic.
 8. The method as defined in claim 7, wherein saidframework is machined metal.
 9. The method as defined in claim 8,wherein said machined metal is aluminum.
 10. The method as defined inclaim 1, wherein said framework is machined metal.
 11. The method asdefined in claim 10, wherein said machined metal is aluminum.
 12. Themethod as defined in claim 1, wherein said tubular metal blank has onlytwo end openings.
 13. The method as defined in claim 1, wherein saidpredetermined shape has an axial profile and including: (h) preformingsaid tubular metal blank into an axial profile generally confirming tosaid axial profile of said predetermined shape.
 14. The method asdefined in claim 13, including: (i) heating said tubular metal blankbefore forming said tubular metal blank.
 15. The method as defined inclaim 14, wherein said heating is by passing a heating current throughsaid tubular metal blank.
 16. The method as defined in claim 1,including: (h) heating said tubular metal blank before forming saidtubular metal blank.
 17. The method as defined in claim 16, wherein saidheating is by passing a heating current through said tubular metalblank.
 18. The method as defined in claim 1, wherein said inductionheating being varied along the length of said tubular metal blank tomodulate the temperature pattern along said length.
 19. The method asdefined in claim 18, wherein an alternating current powers said axiallyspaced conductors and said variation is by varying the frequency of saidalternating current.
 20. The method as defined in claim 18, wherein analternating current powers said axially spaced conductors and saidvariation is by varying the heating time of said alternating current.21. The method as defined in claim 18, wherein said variation is byvarying the distance said axially spaced conductors are from said shell.22. The method as defined in claim 18, wherein said variation is byvarying the spacing between said axially spaced conductors.
 23. Themethod as defined in claim 18, wherein an alternating current powerssaid axially spaced conductors and said variation is by varying thepower of said alternating current.
 24. The method as defined in claim18, wherein said predetermined shape has an axial profile and including:(h) preforming said tubular blank into an axial profile generallyconfirming to said axial profile of said predetermined shape.
 25. Themethod as defined in claim 24, including: (i) heating said tubular metalblank before forming said tubular metal blank.
 26. The method as definedin claim 18, wherein said heating is by passing a heating currentthrough said tubular metal blank.
 27. The method as defined in claim 18,including transferring said formed component into a quench station andquenching said component along the axial length of said component. 28.The method as defined in claim 27, including varying said quenchingalong said axial length.
 29. The method as defined in claim 28, whereinsaid quench variation is by varying the flow rate of quenching fluidalong said length.
 30. The method as defined in claim 28, wherein saidquench variation is by changing location of said quenching operationalong said length.
 31. The method as defined in claim 1, includingtransferring said formed component into a quench station and quenchingsaid component along the axial length of said component.
 32. The methodas defined in claim 31, including varying said quenching along saidaxial length.
 33. The method as defined in claim 32, wherein said quenchvariation is by varying the flow rate of quenching fluid along saidlength.
 34. The method as defined in claim 32, wherein said quenchvariation is by changing location of said quenching operation along saidlength.
 35. The method as defined in claim 31, wherein saidpredetermined shape has an axial profile and including: (h) preformingsaid tubular metal blank into an axial profile generally confirming tosaid axial profile of said predetermined shape.
 36. The method asdefined in claim 35, wherein said heating is by passing a heatingcurrent through said tubular metal blank.
 37. The method as defined inclaim 36, including: (h) heating said tubular metal blank before formingsaid tubular metal blank.
 38. The method as defined in claim 1,including feeding of metal from said tubular metal blank into said shellwhile said blank is being formed.
 39. The method as defined in claim 38,wherein said induction heating is varied along the length of saidtubular metal blank to modulate the temperature/time pattern along saidlength.
 40. The method as defined in claim 39, including transferringsaid formed component into a quench station and quenching said componentalong the axial length of said component.
 41. The method as defined inclaim 40, including varying said quenching along said axial length. 42.The method as defined in claim 1, including sensing the pressure of saidgas in said shell and controlling the gas pressure of the gas forcedinto said tubular metal blank to a preselected value.
 43. The method asdefined in claim 42, wherein said preselected value is in the range ofabout 200-1000 psi.
 44. The method as defined in claim 1, wherein saidhigh pressure is in the range of 200-1000 psi.
 45. The method as definedin claim 44, wherein said induction heating is varied along the lengthof said tubular metal blank to modulate the temperature/time patternalong said length.
 46. The method as defined in claim 45, includingtransferring said formed component into a quench station and quenchingsaid component along the axial length of said component.
 47. The methodas defined in claim 46, including varying said quenching along saidaxial length.
 48. The method as defined in claim 44, includingtransferring said formed component into a quench station and quenchingsaid component along the axial length of said component.
 49. The methodas defined in claim 48, including varying said quenching along saidaxial length.
 50. The method defined in claim 1, wherein said tubularmetal blank is a steel blank capable of metallurgical and physicalchanges by heating and quenching.
 51. The method as defined in claim 50,wherein said tubular metal blank has a thickness in the range of0.040-0.350 inches.
 52. The method as defined in claim 1, wherein saidtubular metal blank is an aluminum blank capable of metallurgical and/orphysical changes by heating and quenching.
 53. A method of forming anelongated tubular metal blank with a length between first and secondends, at least one of said ends being open, into a tubular componenthaving a predetermined outer configuration, said method comprising: (a)plugging said open end or ends of said tubular metal blank; (b) placingsaid plugged tubular metal blank into a cavity with an inner surfacesurrounding said blank and having said predetermined outerconfiguration; (c) forming said tubular metal blank into said componentby inductively heating axial portions along the length of said tubularmetal blank by axially spaced conductors adjacent said cavity whileforcing gas at a high pressure into said plugged tubular metal blankuntil said tubular metal blank conforms to at least a portion of saidinner surface of said cavity to form said component, said inductionheating being varied along the length of said tubular metal blank tomodulate the temperature/time pattern along said length.
 54. The methodas defined in claim 53, wherein an alternating current powers saidaxially spaced conductors and said variation is by varying the frequencyof said alternating current.
 55. The method as defined in claim 53,wherein said variation is by varying the distance said axially spacedconductors are from said shell.
 56. The method as defined in claim 53,wherein said variation is by varying the spacing between said axiallyspaced conductors.
 57. The method as defined in claim 53, wherein analternating current powers said axially spaced conductors and saidvariation is by varying the power of said alternating current.
 58. Themethod as defined in claim 53, including transferring said formedcomponent into a quench station and quenching said component along theaxial length of said component.
 59. A method of forming an elongatedtubular metal blank with a length between first and second ends, atleast one of said ends being open, into a tubular component having apredetermined outer configuration, said method comprising: (a) pluggingsaid open end or ends of said tubular metal blank; (b) placing saidplugged tubular metal blank into a cavity with an inner surfacesurrounding said blank and having said predetermined outerconfiguration; (c) forming said tubular metal blank into said componentby inductively heating axial portions along the length of said tubularmetal blank by axially spaced conductors adjacent said cavity whileforcing gas at a high pressure into said plugged tubular metal blankuntil said tubular metal blank conforms to at least a portion of saidinner surface of said cavity to form said component; (d) transferringsaid formed component into a quench station and quenching said componentalong the axial length of said component; and (e) varying said quenchingalong said axial length, said quench variation is by varying the flowrate of quenching fluid along said length.
 60. A method of forming anelongated tubular metal blank with a length between first and secondends, at least one of said ends being open, into a tubular componenthaving a predetermined outer configuration, said method comprising: (a)plugging said open end or ends of said tubular metal blank; (b) placingsaid plugged tubular metal blank into a cavity with an inner surfacesurrounding said blank and having said predetermined outerconfiguration; (c) forming said tubular metal blank into said componentby inductively heating axial portions along the length of said tubularmetal blank by axially spaced conductors adjacent said cavity whileforcing gas at a high pressure into said plugged tubular metal blankuntil said tubular metal blank conforms to at least a portion of saidinner surface of said cavity to form said component; (d) transferringsaid formed component into a quench station and quenching said componentalong the axial length of said component; and (e) varying said quenchingalong said axial length, said quench variation is by changing locationof said quenching operation along said length.
 61. A method of formingan elongated tubular metal blank with a length between first and secondends, at least one of said ends being open, into a tubular componenthaving a predetermined outer configuration, said method comprising: (a)plugging said open end or ends of said tubular metal blank; (b) placingsaid plugged tubular metal blank into a cavity with an inner surfacesurrounding said tubular metal blank and having said predetermined outershape; (c) forming said tubular metal blank into said component byinductively heating said blank along its length while forcing gas at ahigh pressure into said plugged tubular metal blank until said tubularmetal blank conforms to at least a portion of said inner surface of saidcavity to form said component; (d) transferring said formed componentinto a quench station and quenching said component along said axiallength; (e) varying said quenching along said axial length by varyingthe flow rate of quenching fluid along said length.
 62. The method asdefined in claim 61, wherein said quench variation is by changinglocation of said quenching operation along said length.
 63. The methodas defined in claim 61, wherein said induction heating is varied alongthe length of said tubular metal blank to modulate the temperature/timepattern along said length.
 64. The method as defined in claim 63,wherein an alternating current powers axially spaced conductors in saidinduction heating and said variation is by varying the frequency of saidalternating current.
 65. The method as defined in claim 63, wherein analternating current powers axially spaced conductors in said inductionheating and said variation is by varying the heating time of saidalternating current.
 66. The method as defined in claim 63, wherein saidvariation is by varying the spacing between axially spaced conductors insaid induction heating.
 67. The method as defined in claim 63, whereinan alternating current powers axially spaced conductors in saidinduction heating and said variation is by varying the power of saidalternating current.
 68. The method as defined in claim 61, includingsensing the pressure of said gas in said shell and controlling the gaspressure of the gas forced into said tubular blank to a preselectedvalue.
 69. The method as defined in claim 68, wherein said preselectedvalue is in the range of about 200-1000 psi.
 70. The method as definedin claim 61, wherein said high pressure is in the range of 200-1000 psi.71. The method as defined in claim 61, including forming said cavityfrom a low permeability, rigid material and supporting said cavity in acast non-magnetic material.
 72. The method as defined in claim 71,wherein said rigid material is fused silica.
 73. The method as definedin claim 72, wherein said rigid material is selected from the classconsisting of silicon nitride, silicon carbide, beryllium oxide, boronoxide, and zirconia.
 74. The method as defined in claim 61, wherein saidquenching cools said component to a given temperature above ambient fora time to provide arrested cooling.
 75. A method of forming an elongatedtubular metal blank with a length between first and second ends, atleast one of such ends being open, into a tubular component having apredetermined shape, said method comprising: (a) plugging said open endor ends of said tubular metal blank; (b) placing said plugged tubularmetal blank into a cavity with an inner surface surrounding said tubularmetal blank and having said predetermined outer shape; (c) forcing gasat a high pressure into said plugged tubular metal blank until saidtubular metal blank conforms to at least a portion of said inner surfaceof said cavity to form said component; (d) inductively heating saidtubular metal blank along its length while said tubular metal blank isin said cavity whereby said tubular metal blank is heated before and/orwhile it is formed; and, (e) varying the induction heating along saidlength, said induction heating is by inductors spaced along the axiallength of said tubular metal blank, an alternating current powers saidaxially spaced inductors and said variation is by varying the frequencyof said alternating current.
 76. A method of forming an elongatedtubular metal blank with a length between first and second ends, atleast one of such ends being open, into a tubular component having apredetermined shape, said method comprising: (a) plugging said open endor ends of said tubular metal blank; (b) placing said plugged tubularmetal blank into a cavity with an inner surface surrounding said tubularmetal blank and having said predetermined outer shape; (c) forcing gasat a high pressure into said plugged tubular metal blank until saidtubular metal blank conforms to at least a portion of said inner surfaceof said cavity to form said component; (d) inductively heating saidtubular metal blank along its length while said tubular metal blank isin said cavity whereby said tubular metal blank is heated before and/orwhile it is formed; and, (e) varying the induction heating along saidlength, said induction heating is by inductors spaced along the axiallength of said blank, an alternating current powers said axially spacedinductors and said variation is by varying the heating time of saidalternating current.
 77. A method of forming an elongated tubular metalblank with a length between first and second ends, at least one of suchends being open, into a tubular component having a predetermined shape,said method comprising: (a) plugging said open end or ends of saidtubular metal blank; (b) placing said plugged tubular metal blank into acavity with an inner surface surrounding said tubular metal blank andhaving said predetermined outer shape; (c) forcing gas at a highpressure into said plugged tubular metal blank until said tubular metalblank conforms to at least a portion of said inner surface of saidcavity to form said component; (d) inductively heating said tubularmetal blank along its length while said tubular metal blank is in saidcavity whereby said tubular metal blank is heated before and/or while itis formed; and, (e) varying the induction heating along said length,said induction heating is by inductors spaced along the axial length ofsaid blank, an alternating current powers said axially spaced inductorsand said variation is by varying said power of the alternating current.78. A method of forming an elongated tubular metal blank with a lengthbetween first and second ends, at least one of such ends being open,into a tubular component having a predetermined shape, said methodcomprising: (a) plugging said open end or ends of said tubular metalblank; (b) placing said plugged tubular metal blank into a cavity withan inner surface surrounding said tubular metal blank and having saidpredetermined outer shape; (c) forcing gas at a high pressure into saidplugged tubular metal blank until said tubular metal blank conforms toat least a portion of said inner surface of said cavity to form saidcomponent; (d) inductively heating said tubular metal blank along itslength while said tubular metal blank is in said cavity whereby saidtubular metal blank is heated before and/or while it is formed; and, (e)varying the induction heating along said length, said induction heatingis by inductors spaced along the length of said blank, said variation isby varying the permeability of the flux field of selected spacedinductors, said flux field is varied by a flux concentrator positionedalong the length of said tubular metal blank.
 79. A method of forming anelongated tubular metal blank with a length between first and secondends, at least one of such ends being open, into a tubular componenthaving a predetermined shape, said method comprising: (a) plugging saidopen end or ends of said tubular metal blank; (b) placing said pluggedtubular metal blank into a cavity with an inner surface surrounding saidtubular metal blank and having said predetermined outer shape; (c)forcing gas at a high pressure into said plugged tubular metal blankuntil said tubular metal blank conforms to at least a portion of saidinner surface of said cavity to form said component; (d) inductivelyheating said tubular metal blank along its length while said tubularmetal blank is in said cavity whereby said tubular metal blank is heatedbefore and/or while it is formed; and, (e) varying the induction heatingalong said length, said induction heating is by inductors spaced alongthe length of said blank, said variation is by varying the permeabilityof the flux field of selected spaced inductors, said flux field isvaried by a Faraday shield positioned along said length of said tubularmetal blank.